1. Field of the Invention
This invention relates to a drawworks control system for monitoring and controlling a drawworks including a rotatable drum and bearing a load, particularly to drawworks used for oil well drilling rigs in the performance of various operations on the oil well drilling rig.
2. Description of the Related Art
In the oil and gas drilling arts, it is well known to use a drawworks in connection with the rig or derrick to hold and to raise and lower a drill string and associated equipment into and out of the wellbore. Typically, a traveling block having an appropriate hook or other similar assembly is used for the raising and lowering operations. The traveling block is secured in block and tackle fashion to a secured crown block or other limit fixture located at the top of the rig or derrick. The raising and lowering operation of the traveling block is performed by means of a hoist cable or line, one end of which is secured to the rig floor or ground forming a "dead line`, with the other end being secured to the drawworks proper and forming the "fast line". Although the load bearing assembly could take another form in a particular drilling rig embodiment, for purposes herein all such load bearing assemblies regardless of appearance are, for convenience, referred to as the "traveling block", which term also includes the hook or other attachment means, the associated equipment or other load associated therewith as it moves upwardly and downwardly.
The drawworks includes a rotatable cylindrical drum upon which the cable or fast line is wound by means of a suitable prime mover and power assembly. Typically, the drawworks and traveling block assembly is generally controlled and operated by an operator sometimes referred to as the driller. Thus, in association with the raising of the traveling block, the prime mover is controlled by the operator typically by way of a foot or hand throttle. Similarly, in connection with the lowering operation, the drawworks is supplied with one or more suitable brakes, also controlled by the operator, usually with hand controls. Generally, the primary brake, which typically is a friction brake of either a band or disk type, is supplemented with an auxiliary brake, such as an eddy current type brake or a magnetic brake. The drawworks may also be provided with an emergency brake which can be activated in the event of a power failure to the eddy current brake or when the traveling block exceeds a maximum safe falling speed.
The line or wireline is usually a wire rope or steel cable, although other materials have also been used. The well drill string and associated equipment, even in a moderately deep well is highly flexibly and relatively easy to damage. Controlling the drill string and drill string weight applied to the drill bit (the weight-on-bit or WOB) is very important in drilling efficiency. Also, inasmuch as a typical load borne by the traveling block and assembly can exceed four hundred tons or more, an operational error by the operator or driller or a failure in any of the systems controlling the speed or rate of upward or downward movement of the traveling block could be hazardous and even catastrophic, resulting in damage to equipment, personal injury, and even loss of human life. Damage to equipment also occurs when control of the movement of the traveling block is transferred between the prime mover and the brakes or vice-versa. Typically this transfer causes jerking of the equipment which can damage the traveling block and equipment assembly causing unsafe conditions on the drilling rig.
The method and apparatus of the present invention provides a system for monitoring and controlling a drawworks system for facilitating movement of a load suspended from the drawworks system and providing an improved method of transferring control between the brake arrangement and the prime mover associated with the drawworks system.